Brad Pitt's film company Plan B has the rights to a biopic of the life of Rory Stewart, the Tory MP for Penrith and the Border, with actor Orlando Bloom down to play Stewart. If the film is ever made, yesterday's events in Westminster should provide a key scene. It was the day the dashing Conservative drew a line in the sand against arming the rebels in Libya.

Cameron told the Commons the government "has not ruled out" arming the rebels. Stewart, a former diplomat, warned his boss that it would be a mistake that could damage UK and US relations with our allies in the UN for 20 years if they "mucked about" with the wording of UN Resolution 1973 and used it to arm the rebels.

Cameron and Hague feel they "have to do something" after the rebels were forced into headlong retreat at the first sign of close combat with Gaddafi's forces. Despite the diplomatic coup of Libyan foreign minister Moussa Koussa's defection – what Cameron hopes will be the first of many by Gaddafi's top team - there's no sign of the rebels managing to take Tripoli and overthrow the Mad Dog.

Stewart, whose background includes Eton College, the Army, Oxford University, and the Foreign Office, was a senior coalition official in a province of occupied Iraq in 2003-2004. Stewart, still not 40, published two books on his experiences - The Prince of the Marshes (also published under the title Occupational Hazards) and The Places in Between about his 2002 walk across Afghanistan.

He has quickly become the most widely respected voice at Westminster on the fast developing scene in the Middle East and so far has carefully trodden a thin line of loyalty to David Cameron and Foreign Secretary William Hague. Both men would now be wise to heed his words.

"We have got to stick to the spirit not the letter of the law," Stewart said. "We got a resolution through. We got China, India, Germany and Brazil to back it. We don't want to muck around and be too clever with the terms of it."

In the March 21 debate on UN Resolution 1973, Stewart said the Prime Minister would have to stick hard to his commitment not to put boots on the ground in Libya, "because it is easy for us to say today, 'So far and no further', but all the lessons of Afghanistan are that if we dip our toes in, we are very soon up to our neck."

Ed Miliband, the Labour leader, has been keen to support Cameron's adventure in Libya and has failed to utter any word of warning about arming the rebels, though his friends in the media have seen the danger. Daily Mirror journalist Kevin Maguire tweeted: "Libya mission creep: 1) No Fly 2) No Drive 3) Arm Rebels 4) Boots on the ground?"

Stewart's fears arise partly from the point made by The First Post that the West does not know who the rebels are. The US armed the Mujahideen – led by Osama Bbn Laden – to stop the Russians in their tracks in Afghanistan, only to find them firing at US liberation forces when they invaded to remove the Taliban from power.

The spectre haunting Cameron is that something very simmilar happen again in Libya.

Stewart has also cautioned against Cameron talking about Britain owing a moral obligation to the Libyan people, just because we sold arms to the Libyans in the past. "This is very dangerous, and we must get away from that kind of language and into the kind of language that is humble, that accepts our limits," he said.

Cameron has been sucked into the Blairite world of playing global policeman. He needs to take care, the way things are going, that the film-makers don't ask Steve Coogan to play Cameron in that Stewart biopic. Coogan was in the chamber yesterday for PMQs. Just looking – or researching?